drm/i915: Make sure we don't detect eDP on g4x

We don't support eDP on g4x, so let's not even look at the VBT
to determine the port type, just in case the VBT is bonkers
on some g4x machines and indicates the precense of eDP.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>